Vehicle seats are constantly demanded to have a light weight for fuel consumption improvement in view of energy conservation. The present applicant has hitherto made various proposals on technology related to weight reduction of vehicle seats. As one means among them, the applicant proposed one having a structure in which a three-dimensional knitted fabric is strained as respective cushion members of a seat back part and a seat cushion part (see Patent Document 1), instead of urethane materials which are generally used widely. By use of the strained three-dimensional knitted fabric, sufficient cushioning characteristics (vibration absorbency, impact energy absorbency, and the like) can be obtained despite its thinness, and thus weight reduction of the entire seat can be achieved. Further, as described likewise in Patent Literature 1, the applicant also proposed a contrivance of weight reduction by using a thin material as a frame, with a thickness of 1.0 mm or less in the case of iron-based material, or a thickness of 1.6 mm or less in the case of aluminum.